Strong volcanic earthquakes were detected by the single instrument in operation at Bulusan from 22 October to 3 December (typhoons damaged the remainder of the seismic telemetry network). High-frequency earthquakes with S-P intervals ranging from 2.0 to 2.6 seconds were recorded on 22, 23, and 31 October, and 3 December. The 22 October earthquake was felt at intensity II. Steam emission from the upper fissure remained white with the plume varying from wispy to weak. Thermal measurements showed no anomalous change.

Geological Summary. Luzon's southernmost volcano, Bulusan, was constructed along the rim of the 11-km-diameter dacitic-to-rhyolitic Irosin caldera, which was formed about 36,000 years ago. It lies at the SE end of the Bicol volcanic arc occupying the peninsula of the same name that forms the elongated SE tip of Luzon. A broad, flat moat is located below the topographically prominent SW rim of Irosin caldera; the NE rim is buried by the andesitic complex. Bulusan is flanked by several other large intracaldera lava domes and cones, including the prominent Mount Jormajan lava dome on the SW flank and Sharp Peak to the NE. The summit is unvegetated and contains a 300-m-wide, 50-m-deep crater. Three small craters are located on the SE flank. Many moderate explosive eruptions have been recorded since the mid-19th century.

Information Contacts: PHIVOLCS.